slant 6 to 340

I have a question. I swapping a 6 for a 340 in my 1968 Dart. I have the conversin mount kit from Schumacher that I'm going to use. The instructions say I need to change the torsion bars and brakes, I've got drums. I've had other guys say it's not a bad idea but not absolutey necessary right away. Does anyone have an opinion on this? I'm not planning on tearing up the streets, it's just going to be driven for fun.